Continuous polyester process

ABSTRACT

An atmospheric pressure process for the continuous production of polyester is disclosed wherein a melt of dihydroxy ethyl terphthalate, or its low molecular oligomers, obtained by esterifying terephthalic acid or transesterifying dimethyl terephthalate with ethylene glycol, is intimately contacted with an inert gas to facilitate polymerization and removal of the reaction by-products. The ethylene glycol evolved and the inert gas are recycled.

FIELD OF THE INVENTION

An improved process for the continuous production of polyester atatmospheric pressure is disclosed.

TECHNICAL BACKGROUND

Polyester production from terephthalic acid (TPA) or its esters, such asdimethyl terephthalate (DMT), and glycols is known. This has beenaccomplished by stagewise melt polymerization of the dihydroxy ester ofthe bifunctional carboxylic acid, or low molecular weight oligomers,thereof under successively higher vacuum conditions. In order for thepolymerization to continue to the degree needed for most commercialapplications, the condensation by-products, especially ethylene glycol,must be removed from the reaction system at vacuums as high as 1-3 mmHg. Such processes require costly high vacuum equipment, multistagesteam jets to create the vacuum, and N₂ purged seals and flanges tominimize leakage of air into the system. Condensate from the steam jetsand organic by-products from the system end up as a waste water streamthat requires treatment and contributes to volatile organic emissions tothe air. The present invention provides a less costly polymerizationprocess that can be carried out at atmospheric pressure and in a closedloop configuration that eliminates volatile organic emissions and thewaste water discharge.

U.S. Pat. No. 2,973,341 (Hippe) discloses a continuous process for theproduction of polyester condensate and an improved continuous processfor making polyethylene terephthalate from dimethyl terephthalate andethylene glycol. The process employs liquid dimethyl terephthalate andmixes with it ethylene glycol, in an excess molar ratio of 1.5:1, toform a liquid reaction mixture in a first stage below thetransesterification temperature and then carrying the liquid reactionmixture through three separate temperature controlled stages.Transesterification occurs in the second stage at a temperature of notmore than 197° C.; vaporous reaction products are removed in the thirdstage at 197° C. to 230° C. by passing an inert gas through the liquidreaction mixture; polycondensation occurs in the fourth stage at 230° C.to 270° C. for a period of time sufficient to produce a filament formingpolyethylene terephthalate condensate while again passing an inert gasthrough the liquid reaction mixture. Ethylene glycol by-product can berecovered from the fourth stage and recycled to the second stage of thereaction.

U.S. Pat. No. 3,545,520 (Siclari et al.) discloses an apparatus forstripping substances and lightweight fractions from polymers including ameans for introducing an inert gas counter current to the polymericmaterial with the consequent increase in viscosity of the polymers. Theapparatus permits recycling a portion of the material removed from thevessel so that the material can be recycled into the reaction container.

U.S. Pat. No. 3,469,618 (Siclari et al.) discloses a method forstripping off volatile fractions from polyamides and polyestersinvolving feeding material in the form of droplets or liquid threadsthough an inert gaseous atmosphere, while recirculating that atmosphere.

U.S. Pat. No. 3,110,547 (Emmert) discloses a process for preparing alinear condensation polyester. In one embodiment of the invention, thepolymer is extruded downwardly through a chamber while passing a currentof inert gas,, such as nitrogen, through the reaction vessel at a ratesufficient to keep the glycol partial pressure below 2 mm Hg whilemaintaining a temperature between 300° C. and 400° C. in order torapidly finish the polymer by converting the polymer having a degree ofpolymerization of from about 15 to 35 to a finished polymer with adegree of polymerization of about 70.

U.S. Pat. No. 3,390,135 (Seiner) discloses a continuous process forpreparing polyester wherein the product is contacted with an inert gaswhich has been passed over the product in a countercurrent manner, at aregulated flow, to remove the water of esterification.

SUMMARY OF THE INVENTION

The invention relates to a continuous atmospheric pressure method ofpolymerizing a dihydroxy ester of a bifunctional carboxylic acid, or ofa low molecular weight polymerizable oligomer thereof, to a product witha higher degree of polymerization (DP), in the presence of a polyesterpolymerization catalyst, wherein by-products of the polymerization areremoved from the system by means of an inert gas.

This process provides an improved method for producing linear aromaticpolyesters, especially polyethylene terephthalate (PET), also referredto as polyethylene glycol terephthalate. The bifunctional acid in theproduction of PET is terephthalic acid (TPA). The process involves thecontinuous production of polyethylene terephthalate from terephthalicacid and ethylene glycol by esterification, or from dimethylterephthalate and ethylene glycol by a transesterification stage,followed by polycondensation and polymer finishing stages. The processis conducted at atmospheric pressure or above, thereby avoiding highvacuum equipment and eliminating possible air contamination that causesproduct decomposition and gel formation. The process comprises thefollowing steps:

(a) esterifying terephthalic acid or transesterifying dimethylterephthalate with ethylene glycol to produce dihydroxy ethylterphthalate or its low molecular oligomers, and

(b) intimately contacting dihydroxy ethyl terephthalate, or its lowmolecular weight oligomers, in melt form, with an inert gas at avelocity of 0.2 to 5 ft/sec, so that volatile reaction by-products areremoved continuously by the inert gas and wherein the polymerizationproduct is removed continuously, while the reactants are kept at asuitable temperature to maintain the melt and to continuepolymerization. The above processes are conducted in the presence of apolyester polymerization catalyst.

In a preferred embodiment of the invention, a single stream of inert gasis recycled through a polymer finishing stage, a polycondensation stageand a stage wherein ethylene glycol is recovered for reuse in theprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic flow sheet for the continuous process of theinvention.

FIG. 2 represents one apparatus which is suitable for carrying out thecontinuous polymerization of the invention, wherein material having alower degree of polymerization is converted to material having a higherdegree of polymerization.

DETAILED DESCRIPTION OF THE INVENTION

Polyethylene terephthalate is manufactured in this process by firstreacting terephthalic acid (TPA) or dimethyl terephthalate (DMT) withethylene glycol. If DMT is the starting material, a suitabletransesterification catalyst such as zinc or manganese acetate is usedfor the reaction. Esterified DMT/TPA is polymerized as a melt atatmospheric pressure or above by intimately contacting the melt with astream of inert gas (for example, but not limited to, N₂ or CO₂) toremove the condensation by-products, mainly, ethylene glycol.Preferably, the inert gas is preheated to about polymerizationtemperature or above, prior to its introduction into the polymerizationequipment. It is preferred that the inert gas velocity through thepolymerization equipment be in the range of 0.2 to 5 ft./sec., mostpreferably 0.5 to 2 ft/sec, flowing counter currently to the flow of themelt. The vapor leaving the polymerization equipment (after a finishingstage and a polycondensation stage) is fractionated to recover ethyleneglycol for recycle. The nitrogen stream is then cleaned up and recycled.Thus, the overall process operates as a closed loop system which avoidsenvironmental pollution and integrates ethylene glycol purification andits recycle into the process.

Catalysts for facilitating the polymerization are any one or morepolyester polymerization catalysts known in the prior art to catalyzesuch polymerization processes, such as, but not limited to, compounds ofantimony, germanium and titanium. Antimony trioxide (Sb₂ O₃) is anespecially effective catalyst which may be introduced, for convenience,as a glycolate solution in ethylene glycol. Examples of such catalystsare found in U.S. Pat. Nos. 2,578,660, 2,647,885 and 2,789,772, whichare incorporated herein by reference.

Dihydroxy esters of bifunctional carboxylic acids used in the processesdescribed herein are monomeric compounds that can polymerize to apolymer. Examples of such compounds arebis(2-hydroxyethyl)terephthalate, bis(4-hydroxybutyl)terephthalate,bis(2-hydroxyethyl) naphthalenedioate, bis(2-hydroxyethyl)isophthalate,bis[2-(2-hydroxyethoxy)ethyl]terephthalate,bis[2-(2-hydroxyethoxy)ethyl]isophthalate,bis[(4-hydroxymethylcyclohexyl)methyl]terephthalate,bis[(4-hydroxymethylcyclohexyl)methyl]isophthalate, and a combination ofbis(4-hydroxybutyl) terephthalate and their oligomers. Mixtures of thesemonomers and oligomers may also be used.

By a "polymerizable oligomer" is meant any oligomeric material which canpolymerize to a polyester. This oligomer may contain low molecularweight polyester, and varying amounts of monomer. For example, thereaction of dimethyl terephthalate or terephthalic acid with ethyleneglycol, when carried out to remove methyl ester or carboxylic groupsusually yields a mixture of bis(2-hydroxyethyl) terephthalate, lowmolecular weight polymers (oligomers) of bis(2hydroxyethyl)terephthalate and oligomers of mono(2-hydroxyethyl) terephthalate (whichcontains carbonyl groups). This type of material is referred to hereinas "polymerizable oligomer".

Polyesters produced by the process include, but are not limited to,poly(ethylene terephthalate), poly(1,4-butylene terephthalate),poly(ethylene naphthalenedioate), poly(ethylene isophthalate, poly(3-oxa-1,5-pentadiyl terephthalate), poly(3-oxa-1,5-pentadiylisophthalate), poly[1,4-bis(oxymethyl)cyclohexyl terephthalate] andpoly[1,4-bis(oxymethyl)cyclohexyl isophthalate]. Poly(ethyleneterephthalate) is is an especially important commercial product.

The process avoids high vacuum polymerization processes characteristicof the conventional art. Advantages of the process are a simpler flowpattern, lower operating costs and the avoidance of steam jets, hotwells and atmosphere emissions. The process also has environmentaladvantages due to the elimination of volatile organic emissions andwaste water discharge. Furthermore, polymerization is conducted in aninert environment. Therefore, there is less decomposition and gelformation which results in better product quality. Ethylene glycol andinert as (e.g., N₂ or CO₂) are recycled continuously. The process isdescribed in greater detail with reference to FIGS. 1 and 2.

FIG. 1 is a diagrammatic flow sheet for the continuous process of theinvention. Reactant materials TPA (or its dimethyl ester, DMT) andethylene glycol are supplied continuously to an esterification column(2) for esterification (or transesterification) to DHET and its low DPoligomers. The resulting esterified or transesterified product is anoligomer with a low degree of polymerization (DP). The resulting DP isfrom 1-2 if the starting material is DMT. If TPA is the startingmaterial, the resulting oligomer usually has a higher DP, in the rangeof from about 3-7. The molten reaction product formed in theesterification column (2) is conducted through transfer line (4) to aprepolymerization column (6) for polycondensation. A suitable polyesterpolymerization catalyst, such as Sb₂ O₃, may be added at this point. Theprepolymer, exiting the esterification column with a degree ofpolymerization from 15-30, is conducted through transfer line (8) tofinisher (10) in order to finish the polymer by raising the degree ofpolymerization to about 50 to about 150, preferably about 60 to about100. The finisher (10) is maintained at a temperature greater than about260° C. but not high enough to result in polymer decomposition. Atemperature range of about 270° C. to 300° C. is preferred. Thepolymerization product is continuously removed from the finisher throughline (30). An inert gas, preferably nitrogen, is heated in heater (12)at a temperature of from about 280° C. to 320° C. and is introducedthrough line (14) into the finisher to flow counter current to thedirection of polymer flow in order to remove volatile reactionby-products, primarily ethylene glycol. The inert gas flows through thefinisher (10) and then through line (16) to prepolymerization column (6)removing volatile reaction by-products, which are mainly ethyleneglycol, in that reaction column. The hot inert gas stream containingorganic vapors, which are mainly ethylene glycol with minor amounts ofmethanol, water, and some thermal decomposition products, exits theprepolymerization column through line (18) and enters the glycolrecovery column (20) where glycol is recovered from the stream andrefined without the need for additional external heat. The recoveredglycol is recycled to the esterification column (2) through line (22).The inert gas stream containing the volatile organics, such asacetaldehyde, exits the glycol recovery column through line (24) andenters an adsorption bed (26), such as an activated carbon bed, whereinthe organic volatiles are adsorbed producing a clean nitrogen streamwhich can be heated and returned to the finisher (10). Thus, thenitrogen is employed in a closed loop and all processing equipment isoperated at atmospheric pressure (or above, as is necessary to ensurethe flow of nitrogen through the equipment in the loop). The inert gasflowing in the polymerization equipment (6) and (10) has a velocity ofbetween about 0.2 to 5 ft/sec, preferably 0.5 to 2 ft/sec. The quantityof inert gas introduced into the system is sufficient so that thepartial pressure of the by-products is maintained below the equilibriumpressure of the by-products with the melt in order to provide for thecontinuous polymerization. The quantity of inert gas is between about0.2-0.5 pounds for each pound of polyethyene terephalate produced. Theadsorption bed (26) can be purged to remove the adsorbed products. Theadsorbed products are transfered by line (28) to a combustion device,such as a boiler, (not shown) where they are converted to carbon dioxideand water by combustion thus completing an environmentally clean,emissions free process.

FIG. 2 illustrates one apparatus which is suitable for carrying out thecontinuous polymerization of the invention particularly for use with thehigh viscosity material and degree of polymerization encountered in thefinisher (10) of FIG. 1. It consists of a horizontal, agitatedcylindrical vessel (32). The esterified DMT or TPA, or a low molecularweight oligomer thereof, is continuously introduced as stream (34) atone end of the vessel (32) and a preheated inert gas, such as nitrogen,is continuously introduced as stream (38) at the other end, so as toprovide a counter current flow to the polymer flow nitrogen stream (38)carrying reaction by-product vapors, mostly ethylene glycol, leaves asstream (40). The polymerized product, polyethylene terephthalate, isremoved as stream (36). The flow rates of streams (34) and (36) arecoordinated to be equivalent to each other and controlled so as toprovide the desired inventory of the melt in the finisher, usually aboutequivalent to 1 to 2 hours times the flow rate, with melt level at about1/3 to 1/2 the height of the vessel. The quantity of nitrogen introducedinto the system is sufficient so that the partial pressure of theevolving reaction by-products is maintained at less than the equilibriumpressure of the by-products with the, for example,poly(ethylene)terephthalate (PET) melt, so as to provide adequatedriving force to remove ethylene glycol from the melt into the gasstream. The diameter of the vessel is designed so that the superficialvelocity of the inert gas stream is about 0.5 to 2 ft/sec.

The vessel is equipped with an agitator (42) which can be rotated at acontrolled speed. The mechanical design of the agitator is such that

(a) the walls o the vessel are wiped;

(b) a large interfacial area of at least 10 ft² /ft³, preferably greaterthen 30 ft² /ft³, is created;

(c) the surface area is renewed frequently; and

(d) good mixing is provided.

One design which achieves the above specified criteria, is a rotatingdisc contactor consisting of several discs mounted on a shaft (in afashion similar to that in conventional continuous polymerizers) but thediscs in this design are sieve plates, with large open area, which allowwell distributed cross flow of the inert gas vapors.

EXAMPLES 1-9

Examples 1-9 were conducted in a test tube heated to 280° to 295° C. byplacing it in a temperature controlled sand bath. The test tub wasequipped with means to introduce preheated N₂ at a controlled rate nearthe bottom and an outlet was created near the top of the test tube toallow N₂ to exit. Except for Example 9, 5 g samples of monomer, preparedat a DuPont commercial plant site by transesterifying DMT with EG, wereplaced in the test tube along with 180 to 1600 ppm of antimony, added asa Sb₂ O₃ catalyst. The catalyst level was not found to affect thepolymerization rate significantly and higher levels led to greyishdiscoloration of the product. Therefore, except for Examples 3, 5, and 6which had catalyst levels of 1600, 400 and 900 ppm, respectively, allother Examples were at lower catalyst levels as shown in Table 1. InExample 9, a 10 g sample was employed and a Mn catalyst used fortransesterification was rendered inactive by reacting with phosphoricaid, before adding the antimony catalyst. This also did not effect thekinetics measurably.

In Examples 8 and 9, the temperature was ramped from 230° C. to 285° C.over a 10 to 15 minute period. This allowed the initial polymerizationto occur at lower temperatures and minimized volatilization of the lowDP oligomers into the N₂ stream.

When the monomer melted in the tube, N₂ was introduced at a flow ratesuch that the superficial gas velocity was in the range expected for acommercial scale operation. The nitrogen velocities employed are shownin Table 1. For the examples where a range of velocities is shown, suchas 0.2-0.6 ft/sec in Example 9, it means that the velocity was at thelower value at the start of the reaction and gradually increased to thehigher value as the polymerization proceeded. N₂ was introduced belowthe melt causing the melt to lift up and allowing it to fall along thetube walls to create interfacial area (estimated at >30 ft² /ft³), andprovide surface renewal and good mixing. Experiments were conducted for12 to 105 minutes and the resulting PET product was analyzed formolecular weight distribution by GPC. The number average degree ofpolymerization calculated from GPC data for each sample are shown inTable 1. The values were independently confirmed by measurements ofintrinsic viscosity.

                  TABLE 1                                                         ______________________________________                                               POLYMERI-             N.sub.2  NO                                      EX-    ZATION       CATALYST VELOCITY AVG.                                    AMPLE  Time (Min.)  ppm Sb   ft./sec. DP                                      ______________________________________                                        1      12           225      0.3-0.6  24                                      2      21           180      0.3-1.0  44                                      3      21           1600     0.3      39                                      4      39           225      0.3-1.3  54                                      5      39           400      0.6      54                                      6      42           900      0.6      57                                      7      60           225      0.3-1.0  64                                      8      105          200      0.2-1.9  182                                     9      90           280      0.2-0.6  70                                      ______________________________________                                    

EXAMPLE 10

Polymerization of the same monomer used in Example 9 was studied on amicrobalance apparatus in a stream of nitrogen in order to determine theimpact of nitrogen velocity on mass transfer. A small sample, 63.6 mg,was suspended in a heated glass tube having a 25 mm inside diameterthrough which nitrogen flowed at a rate of 330 cc/min. Temperature ofthe sample was monitored by a thermocouple mounted close to the sample,while controlling the heat input to the glass tube. The progress ofpolymerization was monitored by observing the weight loss due to theevolution of reaction by-product, ethylene glycol.

The sample was heated to 288° C. and then held at that temperature for90 minutes while maintaining the nitrogen flow rate. The velocity ofnitrogen in the glass tube was calculated as 0.077 ft/sec. Due to thesmall size of the sample, there was a very large surface to volumeratio, estimated at over 180 ft² /ft³. In spite of such a large area(several times that of Examples 1-9) the rate of polymerization was slowdue to the low nitrogen velocity. At the end of 90 minutes the polymerobtained and analyzed by GPC had a number average DP of only about 14.The need for adequate nitrogen velocity was confirmed by thisexperiment.

EXAMPLE 11

The same monomer used in Example 9 was polymerized in a laboratoryapparatus of the type shown in FIG. 2 which was constructed to operateunder the conditions disclosed in Example 12 for a commercial scaleoperation.

The apparatus consisted of a 6 inch glass tube with an inside diameterof 1 inch which was placed in a tube furnace equipped with temperaturecontrol. The tube was fitted with an agitator of 1/8 inch diametercoiled aluminum wire which provided mixing, surface renewal and wipingof the inside tube wall. The agitator was rotated by use of a motorhaving a variable speed gear reducer. It is estimated that the deviceprovided a surface area of about 60 ft² /ft³ of the melt. The polymermelt temperature was monitored by means of a thermocouple inserted intothe tube at each of its two ends.

The tube was filled with 37.6 g of monomer and placed in the furnace.The furnace temperature was raised to a sufficient temperature to meltthe monomer. When the monomer was molten, the agitator was started andpreheated nitrogen was flowed at a velocity of about 0.5 ft/sec throughthe tube.. The temperature set point was then raised to 290° C. toeffect polymerization. When the melt temperature inside the tube reached290° C., the velocity of the nitrogen was raised to 1.1 ft/sec.Polymerization was continued for 90 minutes while controlling theseoperations under the above stated conditions. The actual temperaturenear the nitrogen outlet end varied from around 270° to 299° C. Theagitator speed was initiated at 15 RPM, but was reduced to 8 RPM afterabout 20 minutes and then further reduced to around 3-4 RPM afteranother 20 minutes as the melt became more viscous.

At the end of 90 minutes of polymerization, two samples of the resultingPET were analyzed by GPC. The number average DP was calculated to be 79and 89, respectively. This is higher than the typical value required foryarn and staple use.

To check the feasibility of higher nitrogen velocities, the velocity wasraised to 1.45 ft/sec during the last 3 minutes of operation. No polymercarryover was observed. Just before shutting down, the velocity wasincreased to over 3 ft/sec and was found to be feasible.

EXAMPLE 12

Example 12 illustrates the process of the invention for operatingcontinuously a commercial scale of approximately 100 million pounds peryear. Referring to FIG. 1, about 12,150 lbs/hr of prepolymer ofapproximately 20 DP are fed to finisher (10), maintained at between285°-295° C., and contacted counter currently with a stream of nitrogenheated to about 300° C. and flowed at a rate of 1000 standard cubic feetper minute (SCFM). The flow rate is equivalent to 0.39 of nitrogen perpound of PET produced. The finisher is 7 ft in diameter and 21 ft long.Polyethylene terephthalate, polymerized to a number average DP of 81, iswithdrawn at a rate of 12,000 lbs/hr through line (30) while the levelin the finisher is controlled such that about 1/3 of the finisher volumeremains filled with polymer melt. The melt inventory is thus equivalentto about 100 minutes or 12/3 hours of PET throughput rate. The finisher(10) is equipped with an agitator to provide an interfacial area ofabout 50 square feet per cubic foot of the melt. It provides frequentsurface renewal and good mixing of the melt. The superficial gasvelocity of the nitrogen stream is 1.2 ft/sec under the actual operatingconditions. The nitrogen stream leaving the finisher (10) through line(16) contains approximately 150 pounds of the ethylene glycol evolved inthe finisher The partial pressure of ethylene glycol in the stream isabout 11 mm Hg.

The nitrogen stream leaving the finisher (10) through line (16) is thenfed to the prepolymerizer (6) to provide counter current contact withthe esterification product of about 1.5 average DP, produced bytransesterification of DMT with ethylene glycol, entering theprepolymerizer (6) through line (4) at a rate of about 14,550pounds/hour.

The prepolymerizer tower is 6 ft in diameter and 30 ft high. Theinterior of the tower is designed so as to provide intimate stagedcontact between the melt and the nitrogen vapor such that the hold uptime of the melt in that column is about 20 minutes or 1/3 hour. Thetotal time for polymerization, including the 12/3 hours in the finisheris thus about 2 hours or less. The prepolymerizer is operated at 280° C.A somewhat lower temperature may be maintained at the top of the towerto minimize volatization of the lower molecular weight oligomers. Thenitrogen velocity in the prepolymerizer is about 1 ft/sec near thebottom of the tower and about 1.4 ft/sec near the top of the tower.

The hot nitrogen vapors exit the prepolymerizer (6) through line (18)containing about 2550 pound of ethylene glycol, along with small amountsof other components, such as very low DP oligomers, methanol from theend groups left unreacted during transesterification and minutequantities of high volatile organics, such as acetaldehyde, which may bepresent. The nitrogen stream is fed to the bottom of the ethylene glycolrecovery column (20) through line (18). The column is 4 ft in diameterand the nitrogen velocity averages about 1.8 ft/sec. Heat is removed atthe top of the column to cool the nitrogen to near the ambienttemperature. Essentially all the the ethylene glycol is condensed andleaves the bottom of the column through line (22) as a hot liquid streamof about 150° C. It is recycled through line (22) to the esterificationcolumn (2).

The small amount of oligomers entrained with the nitrogen stream leavingthe prepolymerizer (6) react with the large excess of glycol at thebottom of the EG recovery column, reverting back to the monomer and arerecycled along with the glycol stream to the esterification column. Theuncondensed organics, such as acetaldehyde leave the EG recovery columnalong with the nitrogen through line (24) and are fed to an adsorptionbed (26) of activated carbon. Volatile organic vapors are absorbed onthe bed thus cleaning up the nitrogen stream. The nitrogen stream isheated to about 300° C. and recycled to the finisher. The adsorption bed(26) is periodically purged, when it nears saturation, to removeadsorbed organics which are sent to the boiler house and converted tocarbon dioxide and water. A small amount of nitrogen may be purged fromthe nitrogen loop, and replenished with an equivalent amount of freshnitrogen to keep the levels of impurities in the loop low. Such anitrogen purge may be used for the periodic purging of the adsorptionbed.

What is claimed is:
 1. A process for preparing a linear condensationpolyester by the continuous polymerization of a dihydroxy ester of abifunctional carboxylic acid, or low molecular weight oligomer thereof,with the evolution of volatile reaction by-products including a glycol,to form a product with a higher degree of polymerization, the processconducted at atmospheric pressure or above, comprising contacting thedihydroxy ester of a bifunctional carboxylic acid, or low molecularweight oligomer thereof, in melt form, in the presence of a polyesterpolymerization catalyst, with an inert gas flowing in the process at avelocity of 0.2 to 5 ft/sec, continuously removing volatile reactionby-products by the inert gas and, removing polymerization productcontinuously, while the reactants are maintained in melt form.
 2. Aprocess for the continuous production of polyethylene terephthalate fromterephthalic acid and ethylene glycol by esterification followed by theprocess stages of polycondensation and polymer finishing, the processconducted at atmospheric pressure or above, comprising:(a) esterifyingterephthalic acid with ethylene glycol to produce dihydroxy ethylterphthalate or its low molecular oligomers, (b) intimately contactingdihydroxy ethyl terephthalate, or its low molecular weight oligomers, inmelt form, with an inert gas flowing in the process at a velocity of 0.2to 5 ft/sec, so that volatile reaction by-products, including ethyleneglycol, by the inert gas which is recycled in the system, andcontinuously removing the polymerization product, while the reactantsare maintained in melt form, said process conducted in the presence of apolyester polymerization catalyst.
 3. A process for the continuousproduction of polyethylene terephthalate from dimethyl terephthalate andethylene glycol by transesterification followed by the process stages ofpolycondensation and polymer finishing, the process conducted atatmospheric pressure or above, comprising:(a) transesterifying dimethylterephthalate with ethylene glycol to produce dihydroxy ethylterphthalate or its low molecular oligomers, (b) intimately contactingdihydroxy ethyl terephthalate, or its low molecular weight oligomers, inmelt form, with an inert gas flowing in the process at a velocity of 0.2to 5 ft/sec, continuously removing volatile reaction by-products,including ethylene glycol, by the inert gas which is recycled in thesystem, and continuously removing the polymerization product, while thereactants are maintained in melt form and the process is conducted inthe presence of a polyester polymerization catalyst.
 4. The process ofclaim 1 or claim 2 or claim 3 wherein the catalyst is selected fromcompounds of antimony, germanium and titanium.
 5. The process of claim1, claim 2 or claim 3 wherein the inert gas is preheated to aboutpolymerization temperature or above polymerization temperature prior tocontacting it with the melt.
 6. The process of claim 1 or claim 2 orclaim 3 wherein the inert gas in the system flows counter current to theflow of the melt.
 7. The process of claim 1, claim 2 or claim 3 whereinthe quantity of the inert gas introduced into the system is sufficientto keep the partial pressure of the by-products at less than theequilibrium pressure of the by-products with the melt.
 8. The process ofclaim 1, claim 2 or claim 3 wherein the volatile reaction by-productsare recovered and the inert gas is continuously recycled for reuse inthe process.
 9. The process of claim 2 or claim 3 wherein a singlestream of inert gas is recycled through the polymer finishing stage, apolycondensation stage and a stage wherein ethylene glycol is recoveredfor reuse in the process.
 10. The process of claim 1 wherein thedihydroxy ester of a bifunctional carboxylic acid is selected from thegroup consisting of bis(2-hydroxyethyl) terephthalate,bis(4-hydroxybutyl)terephthalate, bis(2-hydroxyethyl) naphthalenedioate,bis(2-hydroxyethyl) isophthalate,bis[2-(2-hydroxyethoxy)ethyl]terephthalate,bis[2-(2-hydroxyethoxy)ethyl]isophthalate,bis[(4-hydroxymethylcyclohexyl)methyl]terephthalate,bis[(4-hydroxymethylcyclohexyl)methyl]isophthalate,bis(4-hydroxybutyl)terephthalate, and oligomers ofbis(4-hydroxybutyl)terephthalate.
 11. The process of claim 1, 2 or 3wherein the inert gas is selected from N₂ and CO₂.
 12. The process ofclaim 2 or claim 3 wherein the temperature of the finishing stage is270° C. to 300° C.